The following description of background art may include insights, discoveries, understandings or disclosures, or associations together with dis-closures not known to the relevant art prior to the present invention but provided by the invention. Some such contributions of the invention may be specifically pointed out below, whereas other such contributions of the invention will be apparent from their context.
In recent years, the phenomenal growth of mobile Internet services and proliferation of smart phones and tablets has increased a demand for mobile broadband services, and hence more network capacity is required. One possibility to increase network capacity and to provide new proximity based services to end users and/or different service providers is to deploy in addition to wide area systems local area systems providing device-to-device (D2D) communications. The device-to-device communication may be scheduled in a distributed manner or in a centralized (network assisted) manned. Regardless of the scheduling manner, channel state information (CSI) between devices is needed for scheduling, or at least to find out whether or not the devices are close enough for direct communication. Since the device-to-device communication may be implemented as a half-duplex communication, in which a device may either transmit or receive over a spectrum, receiving and sending phases of sounding signals, for example, needs be coordinated.